Comparative in vitro and in vivo studies using a bioactive poly(ε-caprolactone)-organosiloxane nanohybrid containing calcium salt

Comparative in vitro and in vivo studies were conducted using a bioactive poly(ε-caprolactone)-organosiloxane nanohybrid containing calcium, which was prepared by sol-gel method. The behavior of human bone marrow stromal cells (hBMSCs) during in vitro osteogenic differentiation were evaluated on poly(ε-caprolactone)-organosiloxane nanohybrid and poly(ε- caprolactone)-organosiloxane nanohybrid coated with apatite, which mimicked in vivo events. hBMSCs cultured on tissue culture plates (TCPs) were used as a control. For comparative studies, in vivo testing was also conducted using poly(ε-caprolactone)-organosiloxane nanohybrid and poly(ε- caprolactone)-organosiloxane nanohybrid coated with apatite in the diaphyseal bone defects of rabbit tibiae. Initial attachments and early proliferations of hBMSCs onto poly(ε-caprolactone)-organosiloxane with or without the apatite layer were comparable those onto TCPs. However, the late proliferation and the osteogenic differentiation activities on poly(ε-caprolactone)- organosiloxane nanohybrid were significantly lower than the hybrid coated with apatite or TCPs. These results were caused by the delayed detachment of hBMSCs induced by the upward growth of spire-shaped apatite granules on the flat apatite layer through mixed nucleation (heterogeneous and homogeneous nucleation) and growth of apatite crystals during cell culture. However, the poly(ε-caprolactone)-organosiloxane nanohybrid showed excellent osteoconductivity as same as poly(ε-caprolactone)-organosiloxane nanohybrid coated with apatite in vivo even though the cell testing results in vitro were poor. This discrepancy can be explained by the difference in initial degree of supersaturation of apatite in cell culture medium and buffering ability between cell culture medium and body fluid with respect to calcium, which directly affects the nucleation mechanism of apatite crystals and the morphology of grown apatite granules. These findings imply that much attention is required and an optimal method should be used to assess cell responses in vitro. Our results suggest that precoating the apatite layer before in vitro testing is desirable for bioactive materials that release calcium quickly and in large amount because this treatment can more closely mimic in vivo events.